1. Field of the Invention
This invention relates to a method of producing a high strength radial type ceramic turbine rotor which is free from cracks in the proximity of a connection between a hub and a blade portion on an air exit side of the rotor.
2. Description of the Prior Art
Recently, silicon ceramic materials such as silicon nitride, silicon carbide, sialon and the like have been noticed as structural materials for gas turbine, diesel engines and the like, because of their light weight and superior heat and thermal shock resistance. Particularly, radial type turbine rotors made of these ceramic materials have been highlighted for applications to gas turbine rotors or turbo charger rotors for automobiles, because these ceramic rotors are lighter and are capable of being used at much higher temperatures than metal turbine rotors and are superior in thermal efficiency to the metal rotors. Moreover, because the radial type turbine rotors are complicated in shape, they are usually molded by an injection molding process, or the like, which forces the ceramic materials into narrow or curved portions or corners of the complicated shapes.
In order to form the rotors by an injection molding of the ceramic materials, a great amount of plasticizer such as resin, wax or the like must be added into the ceramic material. When the injection molded ceramic rotor body is heated or sintered to remove the resin or wax, the shape in the proximity of a connection between a hub and a blade portion on an air exit side so rapidly changes that the resin or wax added as the plasticizer is not uniformly removed. Accordingly, the molded body is not uniform in density, so that locally different shrinkages occur in the body during sintering, resulting in tensile forces which lead to cracks, particularly in the connection between the hub and blade portion on the air exit side.